Calenders are employed in the papermaking industry to improve or modify the surface finish of a paper web. Calenders can also be used to modify the thickness of the paper web or to even out the thickness along or across the web.
A major function of paper calenders is to improve the surface finish of paper which can improve both its appearance and printability. A calender functions by employing pressure to smooth the surface of a paper web as it passes the rolls. One or more of the rolls may be heated.
Supercalenders are comprised of multiple rolls stacked one above the other forming a plurality of nips through which the paper web is passed. Supercalendering is often accomplished on the papermaking machine just prior to forming the paper web into a reel on a winder. Supercalenders are also used in off-machine applications, in combination with reminders, to improve the surface finish of paper on reels. Supercalenders are used to increases the amount of calendering by increasing the length of time the paper web spends transiting a nip by increasing the number of nips. Supercalenders have also employed rolls with compliant covers which form nips of increased length, thus increasing the amount of surface improvement which can be accomplished in passing through each of the nips formed by a supercalender.
Supercalenders have some drawbacks in that the multiplicity of stacked calender rolls adds to the complexity and cost of a calender. Supercalenders also require more time to change calender rolls. Roll change out is often required when the paper grade being processed is changed. Supercalenders can present additional problems upon machine startup or when a paper break occurs because of the multiplicity of nips which are required to be threaded.
For many grades of paper, it has been found that the supercalender can be replaced by a calender of the gloss type. A gloss calender, or a soft calender, typically employs two rolls forming a single nip, or two pairs of rolls forming two nips. The soft calender has one roll with a compliant cover opposed to a hard surfaced heated roll. When the soft nip calender is run at high nip loads of up to 3,000 pounds per linear inch (PLI), one or more soft nip calenders can perform the supercalender function with certain grades of paper. Where the soft nip calender can be used, increased economies are achieved by the greater simplicity of the soft nip calender over that of the supercalender.
Compliant roll covers have typically been manufactured of leather, rubber or specialized synthetic materials such as polyurethane.
There is a significant detriment to using a roll with a compliant surface in a gloss calender. Modern gloss calenders operate with the hard noncompliant roll at a temperature as high as 400.degree. to 500.degree. F., and compliant rolls have limited capability for withstand high temperatures. Hysteresis effects in the compliant roll surface produce heating which aggravates the problem of roll heating. The high temperature and high pressures used in the gloss calender can also create a potential for failure of the roll cover causing it to separate from the metallic shell to which the compliant roll surface is attached. The continual increase in the speed at which paper is formed, from less than 3,000 feet per minute to over 6,000 feet per minute, with the future holding the likelihood that speeds of over 9,000 feet per minute will be reached relatively soon, adds urgency to the need to develop roll covers with reduced hysteresis losses.
One approach to overcoming the limitations of compliant rolls as used in gloss calenders is disclosed in U.S. Pat. No. 5,546,856 to Neider et al. which discloses a compliant belt which is passed through a calender opposite a heated roll. The belt reduces some of the problems produced by hysteresis in a compliant roll in a gloss calender, however at the cost of adding additional complexity and cost to the gloss calender.
The effectiveness of the calender requires the highest temperature possible in the heated roll without overheating the compliant surface. By reducing the hysteresis-generated heat, the amount of heat the compliant roll can accept from the heated roll is increased.
What is needed a roll cover which reduces shear stresses between a roll cover and a steel roll and which reduces internal heat generation due to hysteresis.